Systems for aiding navigation have experienced a significant upsurge in order to cope with the constant increase in air traffic and with the resulting growing work load for the pilot. To reconcile an ever shorter decision time and a general trend to reduce the number of crew members, numerous automated procedures have been developed, with the aim of freeing the crew from routine tasks, and in a general manner, of improving the in-flight performance of an aircraft.
Thus, the known systems for aiding navigation have means for computing trajectories between waypoints defined in a flight plan advised by the pilot. The trajectories, computed at the start of the flight and updated regularly in the course of the flight, are a support for the manoeuvres of the aircraft, which are decided by the pilot or by an automatic piloting system. In the known prior art, the computed trajectory is split between a lateral trajectory, typically a latitude and a longitude, and a vertical profile applied to this lateral trajectory to take into account constraints, for example regarding the relief or the management of fuel consumption.
These existing trajectory computation means turn out nevertheless to be insufficient for certain particular flight phases. Procedures for which a requirement in terms of both altitude and speed of the aircraft is associated with a waypoint have notably made their appearance. The known systems have no navigation aid for this type of procedure, the crew decides only the moment and the manoeuvres to be engaged to allow the aircraft to reach target altitude and speed at the requested waypoint. It is therefore desirable to have means for aiding navigation for these particular flight phases, notably to warn the crew and accompany them in their manoeuvres.
Known among systems for aiding navigation are flight management systems, termed FMS, a functional architecture of which is shown diagrammatically in FIG. 1. In accordance with the ARINC standard 702, they ensure notably the functions of:                Navigation LOCNAV, 170, for performing optimal location of the aircraft as a function of the geo-location means (GPS, GALILEO, VHF radio beacons, inertial platforms, etc.),        Flight plan FPLN, 110, for inputting the geographical elements constituting the skeleton of the route to be followed (departure and arrival procedures, waypoints, etc.),        Navigation database NAVDB 130, for constructing geographical routes and procedures with the help of data included in the bases (points, beacons, interception or altitude legs, etc.),        Performance database, PRF DB 150, containing the craft's aerodynamic and engine parameters,        Lateral trajectory TRAJ, 120, for constructing a continuous trajectory on the basis of the points of the flight plan, complying with the performance of the aircraft and the confinement constraints,        Predictions PRED, 140, for constructing an optimized vertical profile on the lateral trajectory,        Guidance, GUID 200, for guiding in the lateral and vertical planes the aircraft on its 3D trajectory, while optimizing the speed,        Digital data link DATALINK, 180, for communicating with the control centres and other aircraft.        
On the basis of the flight plan FPLN defined by the pilot, consisting of a list of so-called waypoints, a lateral trajectory is determined as a function of the geometry between the waypoints and/or the altitude and speed conditions. On the basis of this lateral trajectory, a prediction function PRED defines an optimized vertical profile taking account of possible altitude, speed and time constraints, if any. Accordingly, the FMS system has performance tables PERFDB 150, which define the modeling of the aerodynamics and engines. The prediction function PRED 140 implements the equations of aircraft dynamics. These equations are based numerically on values contained in the performance tables for computing drag, lift, and thrust. By double integration, the speed vector and the position vector of the aeroplane are deduced therefrom. This predictive computation function is well known to the person skilled in the art and is not repeated in detail here, the method according to the invention uses this function for the construction of a vertical trajectory.
FIG. 2 illustrates the principle of a transition manoeuvre in terms of altitude and speed of an aircraft. Starting from a flight phase carried out at an initial altitude and an initial speed, it is sought to reach a predetermined altitude Ha and a predetermined speed Vd through a transition manoeuvre. FIG. 2 illustrates the case of a manoeuvre making it possible to increase the altitude of the aircraft and to reduce its speed; the principle of the transition also applies to the other possible combinations, ascending or descent, to a higher or lower speed.
The upper graphic presents the evolution of the altitude profile and the lower graphic the evolution of the speed profile between a manoeuvre start point PM and an end point 10 of a transition manoeuvre. For reasons of simplicity, a rectilinear lateral trajectory is represented by the abscissa axis; the transition manoeuvre applies to a non-rectilinear trajectory according to the same principle.
The transition manoeuvre comprises three successive segments:                A first segment 11, commonly called OPEN1, in the course of which the altitude and the speed evolve according to two constant gradients; a substantially constant thrust being delivered by the aircraft's propulsion system. This segment is continued until a predetermined intermediate speed SPD_INT is reached.        A second segment 12, commonly called OPEN2, in the course of which the thrust and the orientation of the aircraft are adapted to ensure a constant speed and an evolution of the altitude according to a constant gradient. This segment is continued until the target altitude 14 of the transition end point 10 is reached.        A third segment 13, commonly called LVL, in the course of which the altitude is maintained constant, and the speed evolves according to a constant gradient until the target speed 15 of the transition end point 10 is reached.        
This type of transition manoeuvre in terms of altitude and speed in three successive segments is a manoeuvre commonly implemented in the course of a flight. Other manoeuvres exist to allow a transition between an altitude and a speed of departure and arrival. For example, by defining an intermediate speed SPD_INT equal to the arrival speed, a two-segment transition manoeuvre is carried out. The method described by the present invention applies according to the same principle to these other transition manoeuvres.
Let us also note that the type of manoeuvre and its constraint parameters, such as for example the intermediate speed value SPD_INT, are input parameters of the previously described prediction function PRED. Typically, the function makes it possible to determine a vertical trajectory by defining a certain number of parameters such as a target altitude and speed, a type of manoeuvre and its constraint parameters.
Nevertheless, there does not exist any automated procedure making it possible to integrate altitude and speed requirements at a waypoint with the vertical trajectory computation. Stated otherwise, the crew alone must determine when to initiate the transition manoeuvre to allow the aircraft to reach target altitude and speed at the desired waypoint.
Such is the case notably when the crew receives instructions from the air traffic control containing a requirement in terms of altitude and/or speed. The following instructions notably, not implemented in a deployment norm but standardized forthwith, are considered to be among these air traffic control instructions:                CLIMB TO REACH [A] BY [B], (i.e. climb to an altitude [A] for a position [B]),        DESCEND TO REACH [A] BY [B], (i.e. descend to an altitude [A] for a position [B]),        REACH [A] BY [B], (i.e. reach an altitude [A] for a position [B]),        CROSS [A] AT [B], (i.e. cross the position [A] at the altitude [B]),        CROSS [A] AT OR ABOVE [B], (i.e. cross the position [A] at an altitude at least equal to [B]),        CROSS [A] AT OR BELOW [B], (i.e. cross the position [A] at an altitude at most equal to [B]),        CROSS [A] AT AND MAINTAIN [B], (i.e. cross the position [A] at the altitude [B] and maintain your altitude),        CROSS [A] BETWEEN [B1] AND [B2], (i.e. cross the position [A] between the altitude [B1] and the altitude [B2]).        